The present invention relates generally to a welding-type system and, more particularly, to a system and method for providing AC welding power using an output inverter having a half-bridge inverter topology.
Welding-type systems, such as welders, plasma cutters, and induction heaters, often include an inverter-based power source that is designed to condition high power to carrying out a desired process. These inverter-based power sources, often referred to as switched-mode power supplies, can take many forms. For example, they may include a half-bridge inverter topology, a full-bridge inverter topology, a forward-converter topology, a flyback topology, a boost-converter topology, a buck-converter topology, and combinations thereof.
Particularly in systems dedicated to driving a welding process, it is sometimes advantageous to provide an alternating current (AC) output power. For example, it is well known that an AC output power is helpful when welding certain metals, such as aluminum. In particular, during a welding process, the aluminum reacts with air and an oxide is formed on the surface of aluminum. This oxide is an electrical insulator and has a higher melting point than the base metal. By periodically reversing the output current from an electrode negative condition to an electrode positive condition, the oxide is removed from the surface and the clean base metal is exposed to the arc.
A wide variety of welding-type power supplies have been developed that are capable of providing an AC output power to drive a welding-type process. In fact, it is relatively easy to re-establish the welding arc when reversing the polarity from electrode positive to electrode negative because the thermionic tungsten electrodes typically used with such processes supply electrons to reignite the arc. However, it is more difficult to re-establish the welding arc when reversing the polarity from electrode negative to electrode positive because the molten weld pool is not a particularly good emitter of electrons until the arc voltage is high enough to initiate cold-cathode emission. Without a high enough restrike voltage, arc rectification can occur during the electrode positive condition.
To this end, the output inverters in such systems are typically designed to provide a relatively high (or excessive) voltage to ensure that sufficient voltage is provided to avoid arc rectification. Accordingly, a full-bridge output inverter topology has frequently been employed in order to ensure that the switches of the output inverter could handle the relatively high voltages required to ensure that sufficient voltage is provided to avoid arc rectification.
It was readily recognized that a half-bridge output inverter topology would be desirable because it would reduce the size, weight, and cost of the output inverter. A half-bridge inverter topology utilizes one diode drop and one transistor drop in the output current path, while a full-bridge inverter topology utilizes one diode drop and two transistor drops in the output current path. However, in may cases, a half-bridge topology was foregone because the cost of a single output transistor and diode that could withstand the peak voltage stress was too high. The peak voltage is determined by the minimum voltage necessary to sustain the arc during current reversal.
Also, during current reversal a clamp or snubber circuit must be present to absorb the energy present in the parasitic inductance of the welding cables. This commutation energy is proportional to the parasitic load inductance times the load current squared and must be transferred to the output snubber during every current reversal. Thus, the power handling requirement of the snubber must be equal to the commutation energy times twice the output switching frequency.
In order to reduce overall system complexity, some systems employ a resistor through which this commutation energy is dissipated as heat. However, this design requires a large resistor that increases the overall system size and weight and generates a significant amount of heat during operation. Accordingly, some systems have used a flyback converter to transfer the energy from the output circuit back to the primary bus of the inverter-based power source. While such designs provide increased system efficiency over simply dissipating the commutation energy across a resistor, it is relatively complex to design and costly to manufacture.
Accordingly, when designing an output inverter topology two design constraints must be balanced. The first is the overall switching complexity of the output inverter and the cost and weight associated therewith. The second is the ability of the output inverter and associated circuits to manage the commutation energy stored in the parasitic inductance of the output/welding cables and any other inductance associated with the output.
Therefore, it would be desirable to have a system and method for reducing the cost, weight, and complexity of the output inverter and managing the commutation energy stored in the parasitic inductance of the output/welding cables and any other inductance associated with the output.